According to Wikipedia, before the Civil War you were generally legally white if your ancestry was more than 3/4 or 7/8 white (depending on state). This was because southern whites knew damn well that most of them would be caught by a one-drop rule. This is from a letter to a Charlottesville newspaper in 1853, during a debate on whether Virginia should introduce such a rule:
[If a one-drop rule were adopted], I doubt not, if many who are reputed to be white, and are in fact so, do not in a very short time find themselves instead of being elevated, reduced by the judgment of a court of competent jurisdiction, to the level of a free negro.
And later, a delegate to South Carolina’s 1895 constitutional convention said this:
If the law is made as it now stands respectable families in Aiken, Barnwell, Colleton, and Orangeburg will be denied the right to intermarry among people with whom they are now associated and identified. At least one hundred families would be affected to my knowledge. They have sent good soldiers to the Confederate Army, and are now landowners and taxpayers. Those men served creditably, and it would be unjust and disgraceful to embarrass them in this way. It is a scientific fact that there is not one full-blooded Caucasian on the floor of this convention. Every member has in him a certain mixture of … colored blood.
[Emphasis added.]
By the 1920s memories had faded (or been deliberately suppressed), and the one-drop rule was introduced.
